1.) Field of the Invention
The invention relates to bearing materials of silicon carbide, a process for their production and their use.
2.) Background Art
Dense sintered SiC has a high hardness, high-temperature strength, high thermal conductivity, thermal shock resistance, oxidation resistance as well as high abrasion and corrosion resistance. It also displays very good tribological behavior, by which is meant the frictional and wear behavior with and without lubrication. For this reason, sintered pure SiC has become established as an almost ideal material for sliding bearings subject to wear, in particular rotating mechanical seals, and is displacing other materials such as aluminum oxide or cemented carbide in these applications. Thus, rotating mechanical seals and sliding bearings made of sintered silicon carbide (SSiC) have been successfully used since the end of the 1970s in pumps which are subject to high corrosive and abrasive wear stresses. Dense sintered SiC has a purity of .gtoreq.98.5% by weight of SiC and has a sintered density of typically 3.10-3.16 g/cm.sup.3, corresponding to a residual porosity of 1-3% by volume. Thanks to its high hardness (Knoop HK.sub.0.1 =2500) and strength (flexural strength: about 400 MN/m.sup.2), sintered SiC is exceptionally resistant to wear by solid particles which are entrained in liquid media. Even in the case of combined abrasive and corrosive wear, this ceramic material maintains its resistance.
Owing to the universal corrosion resistance, the exceptional wear resistance and the good tribological properties, many bearing and seal problems have been able to be solved using densely sintered SiC (commercially available, for example, from Elektroschmelzwerk Kempten under the name EKasic.RTM.D). This material is described, for example, in proc. 10th Int. Pumps Users Symposium, pp. 63-69. In the case of hermetically sealed pumps too, which are becoming increasingly important in the context of strict environmental regulations, the breakthrough came only with media-lubricated sliding bearings made of SSiC.
Many of the sliding wear problems which nevertheless occur in practice are attributable to interruption of ideal, i.e. properly lubricated, running conditions. In such a case, the sliding surfaces of the bearings or seals concerned come into contact with one another resulting in solid-to-solid or dry friction which is shown by a great increase in the coefficient of friction and leads to temperature peaks.
For applications under severe hydrodynamic conditions, material modifications which, as a result of appropriate configuration of the functional surfaces, continue to ensure sufficient stabilization of the hydrodynamic lubricating film even under short-term running conditions of mixed friction and dry running are known:
Elektroschmelzwerk Kempten GmbH (ESK) offers an SiC material having specifically introduced pores (mean pore size about 40 .mu.m) under the name Ekasic.RTM. Tribo 2000. In this material, the pores act as microscopic lubricating pockets in the sliding surface. In the case of a brief breakdown of the hydrodynamic lubricating film, they continue to make some residual lubrication possible.
Furthermore, ESK offers an SiC material containing specifically introduced pores and graphite particles (mean particle size about 60 .mu.m) under the name Ekasic.RTM. Tribo 2000-1. This material displays a distinctly improved running behavior in dynamic rotating mechanical seals having a hard/hard pairing (SiC against SiC) which run under mix and limited friction conditions at high pressure differences.
Materials having specifically introduced pores are described in EP-A-685437. Materials containing coarse graphite particles are described in EP 709352.
Although the known SiC material modifications can provide a successful solution for many applications in the field of bearings and seals, time and again there are critical applications, particularly in the hot water field, where corrosion can occur on the sliding surfaces even in the case of sintered SiC materials.
Applications in the hot water field are, for example, rotating mechanical seals having a hard/soft pairing (SiC against graphite) of a hard/hard pairing (SiC against SiC) for heating and power station pumps under the following use conditions: temperature of the medium: preferably from 50 to 200.degree. C., particularly preferably 60-150.degree. C., pressure difference: preferably 2-20 bar, particularly preferably 5-10 bar, sliding speed: preferably from 2 to 20 m/s, particularly preferably &lt;10 m/s.
In the case of high sliding speeds and unfavorable running conditions, short-term dry running with local temperature peaks of &gt;200.degree. C. in the sliding surfaces can occur, for example, in a rotating mechanical seal. Owing to the good thermal conductivity of SiC, these high temperatures are only reached for a short time in regions close to the surface (hot spots), but these temperature peaks can nevertheless lead to grain boundary corrosion to a depth of about 20 .mu.m. If the SiC microstructure is fine-grained, by which is meant an SiC microstructure having a grain size of &lt;20 .mu.m, tribochemical reactions in the sealing gap can occur in these regions via crystallite pull-out and these can then lead to formation of an SiO.sub.2 layer on the sliding surfaces. These white layers, which are sometimes visible to the naked eye, can alter the sealing geometry until the seal fails.
Although dense, sintered SiC generally copes better with such situations than do other ceramics, there is a need for further-developed SiC bearing materials, particularly for applications in the hot water field.
It is therefore an object of the invention to improve the corrosion resistance in aqueous media under increased thermal stresses.